How sunscreen works

Despite the constant admonishment to use, then reapply, sunscreen every two hours, the entire product category can be confusing. Here's our cheat sheet:

Ultraviolet, or UV, light is the sun's radiation. There are three types of UV light, only two of which reach the Earth and impact skin.

UVA accounts for 95% of the sun's radiation that reaches the Earth's surface. It penetrates the skin and contributes to skin damage, such as wrinkles, and skin cancer.

UVB accounts for the remaining 5% of ultraviolet radiation that reaches the Earth. It is responsible for causing sunburn.

Broad-spectrum sunscreens are designed to protect users from UVA and UVB rays.

A sunscreen's Sun Protection Factor, or SPF, measures only UVB protection. An SPF 15 blocks 94% of UVB rays, SPF 30 blocks 97% and SPF 45 blocks 98%. The American Academy of Dermatology recommends sunscreens with an SPF of 30.

Natural sunscreens use two active ingredients: titanium dioxide and zinc oxide, the latter of which can temporarily whiten skin. Both work by physically blocking the sun's rays.

Traditional sunscreens use chemicals such as oxybenzone and avobenzone to absorb ultraviolet rays and prevent them from causing damage. Avobenzone absorbs UVA; oxybenzone, UVB.

TheU.S. Food and Drug Administration, which oversees sunscreen labeling, will adopt new rules designed to give consumers better information about the effectiveness of over-the-counter sunscreens and, for the first time, UVA protection, in December.